Interconnection of neighboring solar cells on a flexible supporting film

ABSTRACT

A method of fabricating a solar cell assembly comprising a plurality of solar cells mounted on a flexible support, the support comprising a conductive layer on the top surface thereof divided into two electrically isolated portions—a first conductive portion and a second conductive portion. Each solar cell comprises a front surface, a rear surface, and a first contact on the rear surface and a second contact on the front surface. Each one of the plurality of solar cells is placed on the first conductive portion with the first contact electrically connected to the first conductive portion so that the solar cells are connected through the first conductive portion. A second contact of each solar cell is then connected to the second conductive portion by an interconnect. The two conductive portions serve as bus bars representing contacts of two different polarities of the solar cell assembly.

REFERENCE TO RELATED APPLICATIONS

The present application is a division of U.S. patent application Ser.No. 16/998,636 filed Aug. 20, 2020 which was a division U.S. patentapplication Ser. No. 15/868,296 filed Jan. 11, 2018, now U.S. Pat. No.10, 790, 406 which in turn is a continuation-in-part of U.S. patentapplication Ser. No. 14/592,519, filed Jan. 8, 2015, which claimed thebenefit of U.S. Provisional Application No. 61/976,108 filed Apr. 7,2014.

All of the above applications are hereby incorporated by reference intheir entirety.

BACKGROUND OF THE DISCLOSURE 1. Field of the Disclosure

The disclosure relates to the field of photovoltaic power devices.

2. Description of the Related Art

Satellite and other space related applications typically use solar cellsdesigned for use in the space environment, i.e., under the AM0 solarspectrum. Such space-qualified solar cells have a sequence of subcellswith compositions and band gaps that have been optimized to achievemaximum efficiency for the AM0 spectrum, which is different from thecompositions and band gaps of terrestrial solar cells, which areoptimized for the AM1.5 solar spectrum.

Another distinctive difference between space-qualified solar cells andterrestrial solar cells is that a space-qualified solar cell mustinclude a cover glass over the semiconductor device to provide radiationresistant shielding from electrons and protons in the space environmentwhich could damage the semiconductor material, while terrestrial solarcells need not include a cover glass. The cover glass is typically aceria doped borosilicate glass that is typically 4 mils in thickness andattached by a transparent adhesive to the solar cell.

A further distinctive difference between space-qualified solar cellarrays and terrestrial solar cell arrays is that a space-qualified solarcell array utilizes silver-plated metal material for interconnectionmembers, while terrestrial solar cells typically utilize copper wire forinterconnects. In some embodiments, the interconnection member can be,for example, a metal plate. Useful metals include, for example,molybdenum; a nickel-cobalt ferrous alloy material designed to becompatible with the thermal expansion characteristics of borosilicateglass such as that available under the trade designation KOVAR fromCarpenter Technology Corporation; a nickel iron alloy material having auniquely low coefficient of thermal expansion available under the tradedesignation Invar, FeNi36, or 64FeNi; or the like.

A further distinctive difference between space-qualified solar cellarrays and terrestrial solar cell arrays is that a space-qualified solarcell array utilizes welding to provide robust electricalinterconnections between the space-qualified solar cells, whileterrestrial solar cell arrays can utilize solder for electricalconnections. Welding is required in space-qualified solar cell arrays toprovide robust electrical connections that can withstand the widetemperature ranges encountered in space. In contrast, solder joints aretypically sufficient to survive the rather narrow temperature ranges(e.g., about −40° C. to about +50° C.) encountered with terrestrialsolar cell arrays.

Qualification and acceptance testing to ensure that space-qualifiedsolar cells can operate satisfactorily at the wide range of temperaturesencountered in space include high-temperature thermal vacuum bake-outand thermal cycling in vacuum (TVAC) or ambient pressure nitrogenatmosphere (APTC). As used herein, the term “space-qualified” shall meansatisfactory operation under exemplary conditions for vacuum bake-outtesting that include exposure to a temperature of +100° C. to +135° C.(e.g., about +100° C., +110° C., +120° C., +125° C., +135° C.) for 2hours to 24 hours, 48 hours, 72 hours, or 96 hours; and exemplaryconditions for TVAC and/or APTC testing that include cycling betweentemperature extremes of −180° C. (e.g., about −180° C., −175° C., −170°C., −165° C., −150° C., −140° C., −128° C., −110° C., −100° C., −75° C.,or −70° C.) to +145° C. (e.g., about +70° C., +80° C., +90° C., +100°C., +110° C., +120° C., +130° C., +135° C., or +145° C.) for 600 to32,000 cycles (e.g., about 600, 700, 1500, 2000, 4000, 5000, 7500,22000, 25000, or 32000 cycles). See, for example, Fatemi et al.,“Qualification and Production of Emcore ZTJ Solar Panels for SpaceMissions,” Photovoltaic Specialists Conference (PVSC), 2013 IEEE 39^(th)(DOI: 10. 1109/PVSC 2013 6745052).

An additional distinctive difference between space-qualified solar cellarrays and terrestrial solar cell arrays is that in some embodimentsspace-qualified solar cell arrays utilize an aluminum honeycomb panelfor a substrate. In some embodiments, the aluminum honeycomb panel mayinclude a carbon composite face sheet. In some embodiments, the facesheet may have a coefficient of thermal expansion (CTE) thatsubstantially matches the CTE of the Ge layer of the solar cell that isattached to the support. Substantially matching the CTE of the facesheet with the CTE of the Ge layer of the space-qualified solar cell canenable the array to withstand the wide temperature ranges andtemperature cycling conditions encountered in space without cracking.

Photovoltaic devices, such as photovoltaic modules or “CIC” (SolarCell+Interconnects+Coverglass) assemblies, comprise one or moreindividual solar cells arranged to produce electric power in response toirradiation by solar light. Sometimes, the individual solar cells arerectangular, often square. Photovoltaic modules, arrays and devicesincluding one or more solar cells may also be substantially rectangular,for example, based on an array of individual solar cells. Arrays ofsubstantially circular solar cells are known to involve the drawback ofinefficient use of the surface on which the solar cells are mounted, dueto space that is not covered by the circular solar cells due to thespace that is left between adjacent solar cells due to their circularconfiguration (cf. U.S. Pat. Nos. 4,235,643 and 4,321,417).

However, solar cells are often produced from circular or substantiallycircular wafers. For example, solar cells for space applications aretypically multi-junction solar cells grown on substantially circularwafers. These circular wafers are sometimes 100 mm or 150 mm diameterwafers. However, as explained above, for assembly into a solar array(henceforth, also referred to as a solar cell assembly), substantiallycircular solar cells, which can be produced from substantially circularwafers to minimize waste of wafer material and, therefore, minimizesolar cell cost, are often not the best option, due to their low arrayfill factor, which increases the overall cost of the photovoltaic arrayor panel and implies an inefficient use of available space. Therefore,the circular wafers are often divided into other form factors to makesolar cells. The preferable form factor for a solar cell for space is arectangle, such as a square, which allows for the area of a rectangularpanel consisting of an array of solar cells to be filled 100%(henceforth, that situation is referred to as a “fill factor” of 100%),assuming that there is no space between the adjacent rectangular solarcells. However, when a single circular wafer is divided into a singlerectangle, the wafer utilization is low. This results in waste. This isillustrated in FIG. 1, showing how conventionally, out of a circularsolar cell wafer 1000 a rectangular solar cell 1001 is obtained, leavingthe rest of the wafer as waste 1002. This rectangular solar cell 1001can then be placed side by side with other rectangular solar cells 1001obtained from other wafers, thereby providing for efficient use of thesurface on which the solar cells are placed (i.e., a high fill factor):a large W/m² ratio can be obtained, which depending on the substrate mayalso imply a high W/kg ratio, of great importance for spaceapplications. That is, closely packed solar cells without any spacebetween the adjacent solar cells is generally preferred, and especiallyfor applications in which W/m² and/or W/kg are important aspects toconsider. This includes space applications, such as solar power devicesfor satellites.

Space applications frequently use high efficiency solar cells, includingmulti-junction solar cells and/or III/V compound semiconductor solarcells. High efficiency solar cell wafers are often costly to produce.Thus, the waste that has conventionally been accepted in the art as theprice to pay for a high fill factor, that is, the waste that is theresult of cutting the rectangular solar cell out of the substantiallycircular solar cell wafer, can imply a considerable cost.

Thus, the option of using substantially circular solar cells,corresponding to substantially circular solar cell wafers, to produce anarray or assembly of space-qualified solar cells, could in some casesbecome an interesting option. There is a trade-off between maximum useof the original wafer material and the fill factor. FIG. 2 shows howcircular wafers can be packed according to a layout for maximum use ofspace, obtaining a fill factor in the order of 90%. This implies thatless wafer material is wasted than in the case of the option shown inFIG. 1, but also a less efficient use of the surface on which the solarcells are mounted, due to the lower fill factor. A further problem isthat with this kind of layout, the pattern features a hexagonal unitcell 2000 (illustrated with broken lines in FIG. 2), which isnon-optimal for producing a rectangular assembly of solar cells. Thehexagonal unit cell is inconvenient for producing rectangular arrays ofsolar cells because the assembly of solar cells will not fit neatly tothe edges or boundaries of a rectangular panel.

It is also known to enhance the wafer utilization and to reduce thewaste by obtaining an octagonal, instead of rectangular, solar cell froma substantially circular wafer, namely, a rectangular solar cell withcropped corners. However, whereas this approach makes it possible toreduce the waste of wafer material, it is non-optimal from the point ofview of the fill factor, as when the rectangular solar cells withcropped corners are placed in rows and columns to form a solar cellarray, the space where the cropped corners meet is left without solarcell material and is thus not used for the conversion of light intoelectric power.

It is possible to reduce the amount of waste and at the same timeachieve a high fill factor by dividing a circular or substantiallycircular wafer not into two single rectangular, such as square, cell,but into a large number of smaller cells. By dividing a circular orsubstantially circular wafer into a large amount of relatively smallcells, such as rectangular cells, most of the wafer material can be usedto produce solar cells, and the waste is reduced. For example, a solarcell wafer having a diameter of 100 mm or 150 mm and a surface area inthe order of 80 cm² or 180 cm² can be used to produce a large amount ofsmall solar cells, such as square or rectangular solar cells each havinga surface area of less than 5 cm², less than 1 cm², less than 0.1 cm² oreven less than 0.05 cm² or less than 0.01 cm². For example,substantially rectangular, such as square, solar cells can be obtainedin which the sides are less than 10, 5, 3, 2, 1 or even 0.5 mm long.Thereby, the amount of waste of wafer material can be substantiallyreduced, and at the same time a high fill factor can be obtained.

However, the use of a large number of relatively small solar cellinvolves the drawback that for a given effective surface area of thefinal solar cell assembly, there is an increased number ofinterconnections between solar cells, in parallel and/or in series,which may render the process of manufacturing the solar cell assemblymore complex and/or expensive, and which may also render the entirecircuit less reliable, due to the risk for errors due to defectiveinterconnections between individual solar cells.

SUMMARY OF THE DISCLOSURE

A first example of the disclosure relates to a method of preparing asolar cell array for space applications, the method comprising: forminga plurality of III-V compound semiconductor multijunctionspace-qualified solar cells optimized for operation at AM0 includingmetallic bonding pads on the top surface thereof, each space-qualifiedsolar cell of the plurality of space-qualified solar cells comprising afront surface, a rear surface, and a first contact in correspondencewith the rear surface; forming a polyimide film having a thickness of 1mil to 4 mils and a conductive layer having a thickness of 1 micrometerto 50 micrometers attached to the polyimide film in an adhesive-lessmanner to mitigate outgassing, the conductive layer comprising a firstconductive section and a second conductive section separated from thefirst conductive section; forming a conductive bonding material directlyadjacent the first conductive section; positioning each space-qualifiedsolar cell of the plurality of space-qualified solar cells directlyadjacent the first conductive section, or directly adjacent theconductive bonding material directly adjacent the first conductivesection; electrically connecting the first contact of each solar cell ofthe plurality of solar cells directly, or solely through the conductivebonding material, to the first conductive section so that the pluralityof solar cells are connected in parallel through the first conductivesection; disposing a ceria doped borosilicate glass supporting memberthat is 4 mils in thickness on a surface of each of the semiconductorsolar cells; and welding interconnects composed of a silver-platednickel-cobalt ferrous alloy material to the metallic bonding pads oneach solar cell, wherein each space-qualified solar cell of theplurality of space-qualified solar cells is a rectangular orsubstantially square space-qualified solar cell having at least oneIII-V compound semiconductor layer and having a surface section of lessthan 1 cm².

Thereby, manufacturing a space-qualified solar cell assembly comprisinga large amount of solar cells becomes easy: the space-qualified solarcells can simply be placed on the first conductive portion, which canmake up a substantial part of the surface of the support, such as morethan 50%, 70%, 80%, 90%, 95% or more of the total surface of thesupport, so that the contact or contacts at the rear surface of eachsolar cell can be easily connected to the first conductive portion ofthe support, which thus serves to interconnect the solar cells inparallel. The connection between the first contact of eachspace-qualified solar cell and the first conductive portion of the metallayer of the support can be direct and/or through a conductive bondingmaterial. Thus, this approach is practical for creating space-qualifiedsolar cell assemblies of a large amount of relatively small solar cells,such as solar cells obtained by dividing a solar cell wafer having asubstantially circular shape into a large number of individual solarcells having a substantially rectangular shape, for enhanced fill factorand wafer utilization. The first conductive portion is continuous andthus acts as a bus interconnecting the first contacts of the solarcells. In addition, the conductive layer, including the first conductiveportion, can act as a thermal sink for the solar cells.

A second example of the disclosure relates to a method of preparing asolar cell assembly designed for space applications, the methodcomprising: forming a plurality of III-V compound semiconductormultijunction space-qualified solar cells optimized for operation at AM0including metallic bonding pads on the top surface thereof each solarcell of the plurality of solar cells comprising a front surface, a rearsurface, a first contact in correspondence with the rear surface, and asecond contact; forming a polyimide film having a thickness of 1 mil to4 mils and a copper conductive layer having a thickness of 1 micrometerto 50 micrometers attached to the polyimide film in an adhesive-lessmanner to mitigate outgassing, the conducting layer comprising a firstconductive section and a second conductive section separated from thefirst conductive section; forming at least one groove traversing theconductive layer, the groove comprising a plurality of segments, atleast one of said segments extending in parallel with another one ofsaid segments so that the groove electrically isolates the firstconductive section and the second conductive section from each other;forming, within the second conductive section, a plurality ofsubstantially elongated subportions that extend between subportions ofthe first conductive section, wherein the first conductive section has alarger surface section than the surface section of the second conductivesection; forming, at the first contact of each solar cell of theplurality of solar cells, a conductive layer extending over asubstantial portion of the rear surface of each solar cell of theplurality of solar cells; placing each solar cell of the plurality ofsolar cells directly adjacent a conductive bonding material that isdirectly adjacent the first conductive section, and electricallyconnected to the first conductive section using the conductive bondingmaterial, wherein the conductive bonding material is selected to enhanceheat transfer between each solar cell and the first conductive portionand without an intervening conductor member, with the first contact ofeach solar cell of the plurality of solar cells electrically connectedto the first conductive section so that the plurality of solar cells areconnected in parallel through the first conductive section; forming aninterconnect connecting the second contact of each solar cell of theplurality of solar cells to the second conductive section toelectrically connecting each solar cell of the plurality of solar cellsto the second conductive section via the second contact of each solarcell of the plurality of solar cells; disposing a ceria dopedborosilicate glass supporting member that is 4 mils in thickness on asurface of each of the semiconductor solar cells; and weldinginterconnects composed of a silver-plated nickel-cobalt ferrous alloymaterial to the metallic bonding pads on each solar cell, wherein theplurality of solar cells placed on the first conductive section form aplurality of rows of solar cells, each solar cell of the plurality ofsolar cells being connected to a subportion of the second conductivesection extending between two rows of solar cells, and wherein eachsolar cell of the plurality of solar cells is a substantiallyrectangular solar cell having at least one III-V compound semiconductorlayer and having a surface section of less than 1 cm².

By means of features such as one or more of the ones listed above, thefirst and the second conductive portions can be designed for optimizeduse of the surface of the support, for example, for providing a maximumsurface for the placement of space-qualified solar cells, whereby thesecond conductive portion provides for conductive tracks that can, forexample, extend between rows of space-qualified solar cell, so that eachtrack serves for collecting the current produced by, for example, one ortwo rows of space-qualified solar cells. Thus, the first and the secondconductive portions can have sophisticated shapes, including, whenviewed from above, extensions of one of said portions entering recessesin the other one, and vice-versa.

In some embodiments of the disclosure, each space qualified solar cellhas a surface area of less than 1 cm². The approach of the disclosurecan be especially advantageous in the case of relatively smallspace-qualified solar cells, such as space-qualified solar cells havinga surface area of less than 1 cm², less than 0.1 cm² or even less than0.05 cm² or 0.01 cm². For example, substantially rectangular, such assquare, space-qualified solar cells can be obtained in which the sidesare less than 10, 5, 3, 2, 1 or even 0.5 mm long. This makes it possibleto obtain rectangular solar cells out of a substantially circular waferwith reduced waste of wafer material, while the approach of thedisclosure makes it possible to easily place and interconnect a largenumber of said space-qualified solar cells in parallel, so that they, incombination, perform as a larger space-qualified solar cell.

In some embodiments of the disclosure, each space-qualified solar cellis bonded to the first conductive portion by a conductive bondingmaterial. Using a conductive bonding material makes it possible toestablish the connection between the first contact of eachspace-qualified solar cell and the support by simply bonding thespace-qualified solar cell to the support using the conductive bondingmaterial. The conductive bonding material can be selected to enhanceheat transfer between space-qualified solar cell and support.

In some embodiments of the disclosure, the conductive bonding materialis an indium alloy. Indium alloys have been found to be useful andadvantageous, in that indium can make the bonding material ductile,thereby allowing the use of the bonding material spread over asubstantial part of the surface of the support without making thesupport substantially more rigid and reducing the risk of formation ofcracks when the assembly is subjected to bending forces. Preferably,support, space-qualified solar cells and bonding material are matched toeach other to feature, for example, similar thermal expansioncharacteristics. On the other hand, the use of a metal alloy, such as anindium alloy, is advantageous over other bonding material such aspolymeric adhesives in that it allows for efficient heat dissipationinto the underlying conductive layer, such as for example a copperlayer. In some embodiments of the disclosure, the bonding material isindium lead.

In some embodiments of the disclosure, the conductive layer comprisescopper.

In some embodiments of the disclosure, the support comprises a Kapton®film, the conductive layer being placed on the Kapton® film. The optionof using a Kapton® film for the support is practical for, for example,space applications.

In some embodiments of the disclosure, the first contact of eachspace-qualified solar cell comprises a conductive, such as a metal,layer extending over a substantial portion of the rear surface of therespective space-qualified solar cell, preferably over more than 50% ofthe rear surface of the respective space-qualified solar cell, morepreferably over more than 90% of the rear surface of the respectivespace-qualified solar cell. In some embodiments of the disclosure, thefirst contact comprises a conductive, such as a metal, layer coveringthe entire rear surface of the space-qualified solar cell. This helps toestablish a good and reliable contact with the first conductive portionof the conductive layer of the support.

In some embodiments of the disclosure, each space-qualified solar cellcomprises at least one III-V compound semiconductor layer. As indicatedabove, high wafer utilization can be especially advantageous when thespace-qualified solar cells are high efficiency space-qualified solarcells such as III-V compound semiconductor space-qualified solar cells,often implying the use of relatively expensive wafer material.

In some embodiments of the disclosure, the solar cell array for spaceapplications assembly has a substantially rectangular shape and asurface area in the range of 25-400 cm².

Another aspect of the disclosure relates to a solar cell array for spaceapplications comprising a plurality of solar cell arrays for spaceapplication, each of these solar cell arrays for space applicationcomprising a solar cell assembly according to the first aspect of thedisclosure. As indicated above, the solar cell arrays for spaceapplications can advantageously serve as sub-assemblies which can beinterconnected to form a major solar cell array for space applications,comprising, for example, an array of such solar cell arrays for spaceapplications comprising a plurality of strings of such solar cell arraysfor space applications, each string comprising a plurality of solar cellarrays for space applications connected in series. Thus, a modularapproach can be used for the manufacture of relatively large solar cellarrays for space applications out of small space-qualified solar cells,which are assembled to form arrays as described above, whereafter thearrays are interconnected to form a larger array.

BRIEF DESCRIPTION OF THE DRAWINGS

To complete the description and in order to provide for a betterunderstanding of the disclosure, a set of drawings is provided. Saiddrawings form an integral part of the description and illustrateembodiments of the disclosure, which should not be interpreted asrestricting the scope of the disclosure, but just as examples of how thedisclosure can be carried out. The drawings comprise the followingfigures:

FIG. 1 schematically illustrates a prior art arrangement for producing aclosely packed solar cell array out of square space-qualified solarcells obtained from a circular solar cell wafer.

FIG. 2 schematically illustrates how circular solar cells packed toobtain a maximum fill factor imply a hexagonal unit cell for thearrangement of space-qualified solar cells in an array ofspace-qualified solar cells, or a space-qualified solar cell assembly.

FIG. 3 is a perspective view of a support that can be used when carryingout some of the embodiments of the disclosure.

FIG. 4 is a perspective view of the support after a step of cutting ameandering groove traversing a top metal layer of the support.

FIGS. 5A-5D schematically illustrate a series of steps of a process formanufacturing a solar cell assembly using the support of FIG. 4.

FIG. 5E is a circuit diagram of the solar cell assembly of FIG. 5D.

FIGS. 6A-6E schematically illustrate a series of steps of a process formanufacturing a solar cell assembly for space application in accordancewith another embodiment of the disclosure.

FIG. 7 is a schematic cross-sectional view of a portion of a solar cellassembly for space application as per FIG. 5D

DETAILED DESCRIPTION

The present disclosure provides a process for the design and fabricationof a solar cell array panel for space application utilizinginterconnected modular subassemblies. Although principally concernedwith the structure and organization of the modular subassemblies, thesolar cells are essential components of such subassemblies, and thus adiscussion of III-V compound semiconductor solar cells is in order here.

FIG. 3 illustrates an example of a support that can be used in anembodiment of the disclosure. The support comprises an insulatingsupport layer 101 and a conductive metal layer 102 arranged on a topsurface of the support layer 101. In some embodiments of the disclosure,the metal layer 102 is a copper layer, having a thickness in the rangeof from 1 μm and up to 50 μm. In some embodiments of the disclosure, thesupport layer 101 is a Kapton® layer, that is, a polyimide film layer.Preferably the metal layer is attached to the support layer in anadhesive-less manner, to limit outgassing when used in a spaceenvironment. In some embodiments of the disclosure the support layer canhave a thickness in the range of 1 mil (25.4 μm) to 4 mil (101.6 μm). Insome embodiments of the disclosure, a support can be provided comprisingKapton®, or another suitable support material, on both sides of themetal film 102, with cut-outs for the attachment of solar cells andinterconnects to the metal film.

FIG. 4 illustrates the support 100 of FIG. 3 after a step in which partof the metal layer 102 has been removed, by for example etching or laserscribing, whereby a channel or groove 103 is formed traversing the metallayer, separating it into a first conductive portion 108 and a secondconductive portion 107. It can be observed how the two portions arenested with each other: the groove 103 follows a meandering path,whereby the first conductive portion 108 features a set of substantiallyparallel strips connected to each other at one end thereof. The secondconductive portion 107 also comprises a set of strips, extending partlyin parallel with the strips of the first conductive portion, betweenadjacent strips of said first conductive portion. It can be seen how thefirst conductive portion 108 and the second conductive portion 107 areelectrically isolated from each other due to the presence of the groove,which traverses the metal layer from a top surface thereof down to thesupport layer 101.

FIG. 5A schematically illustrates how a plurality of solar cells 104have been attached to the first conductive portion 108. Only five ofthese solar cells 104 are shown in FIG. 5A for simplicity, and in FIG.5A the solar cells 104 have been illustrated substantially spaced fromeach other. However, in practice solar cells 104 are preferably placedclose to each other and all throughout the first conductive portion, soas to optimize space utilization: it is preferred that a substantialpercentage, such as more than 50%, 60%, 70%, 80% or 90%, such as morethan 95%, of the surface of the support 101 is covered by solar cells,so as to provide for an optimized W/m² or W/kg ratio. Each solar cellcomprises a first contact 111 on a rear or bottom surface of the solarcell, as shown in FIG. 7, and a second contact 105 on a front or topsurface of the solar cell. In some embodiments of the disclosure, thefirst contact 111 comprises a metal layer covering the entire rearsurface of the solar cell or a substantial portion of the rear surfaceof the solar cell, and the second contact 105 is placed adjacent to anedge of the front surface of the solar cell 104. The second contact 105has preferably a small surface area to allow the major part of the frontsurface of the solar cell to be an effective surface for the conversionof sunlight into electric power. In FIG. 5A, the second contact 105 isonly shown for one of the solar cells, for simplicity.

The solar cell 104 is preferably attached to the first conductiveportion 108 by a conductive bonding material 112 as shown in FIG. 7,such as a layer of a metal alloy, such as an indium alloy, such as anindium lead alloy. As is easily understood from FIG. 5B and FIG. 7, themetal layer including the first conductive portion 108 serves as a heatsink for the solar cells, and an indium alloy such as indium lead hasappropriate heat conduction characteristics. At the same time, indium isadvantageous as it provides for ductility, thereby reducing the risk forcracks in the bonds between the solar cells and the first conductiveportion 108 when the assembly is subjected to bending forces.

FIG. 5B shows the result of a further step of the process, in which thesecond contact 105 of each solar cell has been connected to the secondconductive portion 107 by a connecting member or interconnect 106 (onlyone of these interconnects 106 is shown in FIG. 5B, for simplicity).

FIG. 5C illustrates the solar cell assembly after the next process step,in which a bypass diode 110 has been attached to the second conductiveportion at a free end of one of the strips. The diode has a rearterminal which is connected to the second conductive portion 107, forexample, by means of an indium alloy.

FIG. 5D illustrates the solar cell assembly after the next process stepin which a bypass diode interconnect 109 is attached to electricallyconnect a top terminal of the bypass diode 110 with the first conductiveportion 108.

FIG. 5E is a circuit diagram of the assembly shown in FIG. 5D, in whichfive solar cells 104 are connected in parallel between two bus bars 107and 108, corresponding to the second and to the first conductiveportions, respectively, and with a bypass diode 110 common to the fivesolar cells. Each solar cell is a multijunction solar cell.

It is clear from the embodiment schematically shown in FIGS. 5A-5D howmany small solar cells, such as solar cells having a surface area ofless than 1 cm², less than 0.1 cm² or less than 0.01 cm², can easily beplaced on the first conductive portion 108 such as on differentsubareas, tracks or strips of the first conductive portion, and bondedto it by bonding their back sides to the first conductive portion usinga conductive bond that connects that first or rear contact of the solarcell to the first conductive portion 108, and how interconnects can beadded to connect the second or upper contacts of the solar cells to thesecond conductive portion 107. One or more bypass diodes can easily beadded, as shown.

Thus, an assembly of a plurality of space-qualified solar cellsconnected in parallel is obtained, and this kind of assembly can be usedas a subassembly, together with more subassemblies of the same kind, toform a larger assembly including strings of series connectedsubassemblies.

FIGS. 6A-6E schematically illustrate the different stages of a processin accordance with another embodiment of the disclosure. In FIG. 6A thesubstrate is shown after elimination of part of the metal layer, so thatthe substrate layer 201, such as a Kapton layer, is covered by a firstconductive portion 208 and a second conductive portion 207. The secondconductive portion 207 comprises three segments following a meanderingpath or a portion of a meandering path, and the first conductive portion208 comprises four major, substantially square, portions interconnectedby three shorts strips. In FIG. 6B, one substantially rectangular solarcell has been placed on each of the square portions of the firstconductive portion 208. FIG. 6C is a perspective view in which thesecond contacts 205 of the solar cells 204 can be observed. FIG. 6Dschematically illustrates how an interconnect 206 has been added toconnect the second contact 205 of one of the solar cells 204 to thesecond conductive portion or busbar 207 (only one such interconnect isshown in the drawing, for simplicity). In FIG. 6E, a bypass diode 210and its interconnect have been added to interconnect the firstconductive portion and the second conductive portion.

Just as in the case of FIGS. 5A-5D, also FIGS. 6A-6E are only intendedto schematically show an embodiment of the disclosure. In practice, thespatial distribution will mostly differ: solar cells are to be packedrelatively close to each other and arranged to occupy most of thesurface of the assembly, so as to contribute to an efficient spaceutilization from a W/m² perspective.

FIG. 7 schematically illustrates layers of a cross-section of a portionof the assembly of the embodiment of FIG. 5D. A Kapton® support layer101 supports copper strips 108 and 107, and a solar cell 104 having abottom metal layer 111 forming a first contact is bonded to the copperstrip 108 by an indium alloy layer 112. A second contact 105 at theupper surface of the solar cell 104 is connected to the copper strip 107by the interconnect 106.

Thus a method of preparing a solar cell array for space applications isdescribed. For example, a method of preparing a solar cell array forspace applications comprises: forming a plurality of III-V compoundsemiconductor multijunction space-qualified solar cells optimized foroperation at AM0 including metallic bonding pads on the top surfacethereof, each space-qualified solar cell of the plurality ofspace-qualified solar cells comprising a front surface, a rear surface,and a first contact in correspondence with the rear surface; forming apolyimide film having a thickness of 1 mil to 4 mils and a conductivelayer having a thickness of 1 micrometer to 50 micrometers attached tothe polyimide film in an adhesive-less manner to mitigate outgassing,the conductive layer comprising a first conductive section and a secondconductive section separated from the first conductive section; forminga conductive bonding material directly adjacent the first conductivesection; positioning each space-qualified solar cell of the plurality ofspace-qualified solar cells directly adjacent the first conductivesection, or directly adjacent the conductive bonding material directlyadjacent the first conductive section; electrically connecting the firstcontact of each solar cell of the plurality of solar cells directly, orsolely through the conductive bonding material, to the first conductivesection so that the plurality of solar cells are connected in parallelthrough the first conductive section; disposing a ceria dopedborosilicate glass supporting member that is 4 mils in thickness on asurface of each of the semiconductor solar cells; and weldinginterconnects composed of a silver-plated nickel-cobalt ferrous alloymaterial to the metallic bonding pads on each solar cell, wherein eachspace-qualified solar cell of the plurality of space-qualified solarcells is a rectangular or substantially square space-qualified solarcell having at least one III-V compound semiconductor layer and having asurface section of less than 1 cm².

In another example, a method of preparing a solar cell assembly designedfor space applications comprises: forming a plurality of III-V compoundsemiconductor multijunction space-qualified solar cells optimized foroperation at AM0 including metallic bonding pads on the top surfacethereof each solar cell of the plurality of solar cells comprising afront surface, a rear surface, a first contact in correspondence withthe rear surface, and a second contact; forming a polyimide film havinga thickness of 1 mil to 4 mils and a copper conductive layer having athickness of 1 micrometer to 50 micrometers attached to the polyimidefilm in an adhesive-less manner to mitigate outgassing, the conductinglayer comprising a first conductive section and a second conductivesection separated from the first conductive section; forming at leastone groove traversing the conductive layer, the groove comprising aplurality of segments, at least one of said segments extending inparallel with another one of said segments so that the grooveelectrically isolates the first conductive section and the secondconductive section from each other; forming, within the secondconductive section, a plurality of substantially elongated subportionsthat extend between subportions of the first conductive section, whereinthe first conductive section has a larger surface section than thesurface section of the second conductive section; forming, at the firstcontact of each solar cell of the plurality of solar cells, a conductivelayer extending over a substantial portion of the rear surface of eachsolar cell of the plurality of solar cells; placing each solar cell ofthe plurality of solar cells directly adjacent a conductive bondingmaterial that is directly adjacent the first conductive section, andelectrically connected to the first conductive section using theconductive bonding material, wherein the conductive bonding material isselected to enhance heat transfer between each solar cell and the firstconductive portion and without an intervening conductor member, with thefirst contact of each solar cell of the plurality of solar cellselectrically connected to the first conductive section so that theplurality of solar cells are connected in parallel through the firstconductive section; forming an interconnect connecting the secondcontact of each solar cell of the plurality of solar cells to the secondconductive section to electrically connecting each solar cell of theplurality of solar cells to the second conductive section via the secondcontact of each solar cell of the plurality of solar cells; disposing aceria doped borosilicate glass supporting member that is 4 mils inthickness on a surface of each of the semiconductor solar cells; andwelding interconnects composed of a silver-plated nickel-cobalt ferrousalloy material to the metallic bonding pads on each solar cell, whereinthe plurality of solar cells placed on the first conductive section forma plurality of rows of solar cells, each solar cell of the plurality ofsolar cells being connected to a subportion of the second conductivesection extending between two rows of solar cells, and wherein eachsolar cell of the plurality of solar cells is a substantiallyrectangular solar cell having at least one III-V compound semiconductorlayer and having a surface section of less than 1 cm².

It is to be noted that the terms “front”, “back”, “top”, “bottom”,“over”, “on”, “under”, and the like in the description and in theclaims, if any, are used for descriptive purposes and not necessarilyfor describing permanent relative positions. It is understood that theterms so used are interchangeable under appropriate circumstances suchthat the embodiments of the disclosure described herein are, forexample, capable of operation in other orientations than thoseillustrated or otherwise described herein.

Furthermore, those skilled in the art will recognize that boundariesbetween the above described operations are merely illustrative. Themultiple units/operations may be combined into a single unit/operation,a single unit/operation may be distributed in additionalunits/operations, and units/operations may be operated at leastpartially overlapping in time. Moreover, alternative embodiments mayinclude multiple instances of a particular unit/operation, and the orderof operations may be altered in various other embodiments.

In the claims, the word ‘comprising’ or ‘having’ does not exclude thepresence of other elements or steps than those listed in a claim. Theterms “a” or “an,” as used herein, are defined as one or more than one.Also, the use of introductory phrases such as “at least one” and “one ormore” in the claims should not be construed to imply that theintroduction of another claim element by the indefinite articles “a” or“an” limits any particular claim containing such introduced claimelement to disclosures containing only one such element, even when thesame claim includes the introductory phrases “one or more” or “at leastone” and indefinite articles such as “a” or “an.” The same holds truefor the use of definite articles. Unless stated otherwise, terms such as“first” and “second” are used to arbitrarily distinguish between theelements such terms describe. Thus, these terms are not necessarilyintended to indicate temporal or other prioritization of such elements.The fact that certain measures are recited in mutually different claimsdoes not indicate that a combination of these measures cannot be used toadvantage.

The present disclosure can be embodied in various ways. The abovedescribed orders of the steps for the methods are only intended to beillustrative, and the steps of the methods of the present disclosure arenot limited to the above specifically described orders unless otherwisespecifically stated. Note that the embodiments of the present disclosurecan be freely combined with each other without departing from the spiritand scope of the disclosure.

Although some specific embodiments of the present disclosure have beendemonstrated in detail with examples, it should be understood by aperson skilled in the art that the above examples are only intended tobe illustrative but not to limit the scope of the present disclosure. Itshould be understood that the above embodiments can be modified withoutdeparting from the scope and spirit of the present disclosure which areto be defined by the attached claims.

1. A method of fabricating a solar cell module comprising: providing aplurality of III-V compound semiconductor multijunction solar cells,each solar cell of the plurality of solar cells comprising a frontsurface, a rear surface, a first metal contact in correspondence withthe rear surface and forming a contact of the first polarity type to thesolar cell, and a metallic bonding pad on the front surface of the solarcell forming a contact of a second plurality type to the solar cell;providing a planar support; forming a conducive layer on the uppersurface of the planar support, the conductive layer comprising a firstconductive section and a second conductive section, each section beingelectrically isolated from each other by at least one groove traversingthe conductive layer, wherein the second conductive section comprises aplurality of substantially elongated subportions at least some of whichextend between subportions of the first conductive section; positioningand conductively bonding each solar cell of the plurality of solar cellsto the first conductive section and not on the second conductive sectionof the planar support such that the first contact of each solar cell ofthe plurality of solar cells is electrically connected to the firstconductive section; and providing a plurality of discrete electricalinterconnects, each discrete electrical interconnect coupling themetallic bonding pad on a respective solar cell to a respective portionof the second conductive section of the conductive layer.
 2. A method asdefined in claim, 1, wherein the second conductive section comprises aplurality of substantially elongated subportions at least some of whichextend between subportions of the first conductive section;
 3. A methodas defined in claim 1, further comprising coupling a bypass diodebetween the first conductive section and the second conductive section.4. A method as defined in claim 3, wherein the bypass diode comprises atop side terminal and a rear side terminal, the bypass diode beingplaced on the second conductive section with said rear side terminal ofthe bypass diode being electrically coupled to the second conductivesection, the top side terminal of the bypass diode being electricallycoupled to the first conductive section.
 5. A method as defined in claim3, wherein the bypass diode comprises a top side terminal and rear sideterminal, the bypass diode being placed on the first conductive sectionwith the rear side terminal of the bypass diode electrically coupled tothe first conductive section, the top side terminal of the bypass diodebeing electrically coupled to the second conductive section.
 6. A methodas defined in claim 1, wherein the plurality of solar cells areelectrically connected in parallel, and each solar cell of the pluralityof solar cells is rectangular or substantially square.
 7. A method asdefined in claim 1, wherein the groove follows a path consisting of aplurality of segments arranged one after the other, starting with afirst segment and ending with a final segment, each segment after thefirst segment extending at a right angle with respect to an immediatelyproceeding segment, wherein each of the segments of the groove hassidewalls that are straight in a direction of the path.
 8. A method asdefined in claim 1, wherein at least one of said segments extends inparallel with another one of said segments.
 9. A method as defined inclaim 1, wherein at least one portion of the groove follows asubstantially meandering path.
 10. A method as defined in claim 1,wherein a total surface area of the first conductive section that facesaway from the planar support is larger than a total surface area of thesecond conductive section that faces away from the planar support.
 11. Amethod as defined in claim 1, wherein the plurality of solar cellsplaced on the first conductive section form a plurality of rows of solarcells, each solar cell of the plurality of solar cells being connectedto a subportion of the second conductive section extending between tworows of solar cells.
 12. A method as defined in claim 1, wherein theconductive bonding material is an indium alloy.
 13. A method as definedin claim 12, wherein the bonding material is indium lead.
 14. A methodas defined in claim 1, wherein the planar support is a polyimide filmand the conductive layer comprises copper.
 15. A method as defined inclaim 1, wherein the first contact of each solar cell of the pluralityof solar cells comprises a conductive layer extending over a substantialportion of the rear surface of each solar cell of the plurality of solarcells.
 16. A method as defined in claim 1, wherein the support comprisesa first terminal of a first polarity type on the top surface thereofcoupled to the first conductive section, and a second terminal of asecond plurality type on the top surface thereof coupled to the secondconductive section.
 17. A method as defined in claim 1, wherein thesecond conductive section comprises a strip extending on the top surfaceof the support having a portion disposed substantially parallel to anedge of each of the solar cells to allow an electrical connection to bemade between the bonding pad on the front surface of the solar cell andthe adjacently disposed portion of the second conductive section.
 18. Amethod as defined in claim 1, wherein the first conductive section andthe second conductive section are interdigitated, with the firstconductive section being connected to a bus bar extending along a firstedge of the support, and the second conductive section being connectedto a bus bar disposed along a second edge of the support.
 19. A methodas defined in claim 1, further comprising: disposing a ceria dopedborosilicate glass supporting member on a surface of each of thesemiconductor solar cells; welding interconnects composed of asilver-plated nickel-cobalt ferrous alloy material to the respectivemetallic bonding pads on the solar cells, wherein the interconnects areelectrically connected to the second conductive section of theconductive layer.
 20. A solar cell module comprising: a plurality ofIII-V compound semiconductor multijunction solar cells, each solar cellof the plurality of solar cells comprising a front surface, a rearsurface, a first metal contact in correspondence with the rear surfaceand forming a contact of the first polarity type to the solar cell, anda metallic bonding pad on the front surface of the solar cell forming acontact of a second plurality type to the solar cell; a planar support;a conductive layer disposed on the upper surface of the planar support,the conductive layer comprising a first conductive section and a secondconductive section, each section being electrically isolated from eachother by at least one groove traversing the conductive layer, whereinthe second conductive section comprises a plurality of substantiallyelongated subportions at least some of which extend between subportionsof the first conductive section; each first metal contact of each solarcell of the plurality of the solar cells being conductively bonded tothe first conductive section and not on the second conductive section ofthe planar support such that the first metal contact of each solar cellof the plurality of solar cells is electrically connected to the firstconductive section; and a plurality of discrete electricalinterconnects, each discrete electrical interconnect coupling themetallic bonding pad on a respective solar cell to a respective portionof the second conductive section of the conductive layer.